X2A66鋁鋰合金固態(tài)相變及微觀組織演變規(guī)律的研究

X2A66鋁鋰合金固態(tài)相變及微觀組織演變規(guī)律的研究Study on Solid Phase Transformation and Microstructure Evolution of X2A66 Al-Li Alloy

X2A66鋁鋰合金是我國自主研發(fā)的第四代新型鋁鋰合金,其具有密度低、比強(qiáng)度高、成型性好等優(yōu)點(diǎn),適宜制造飛機(jī)整體壁板結(jié)構(gòu),能夠?qū)崿F(xiàn)材料自身減重和結(jié)構(gòu)設(shè)計(jì)減重的雙重減重目標(biāo),并且具有結(jié)構(gòu)整體性好、靜強(qiáng)度高等優(yōu)點(diǎn),符合未來航空航天技術(shù)的發(fā)展趨勢,具有廣闊的應(yīng)用前景。研究該合金的熱處理工藝對其組織與性能的影響規(guī)律,從而充分挖掘其潛在的性能,可進(jìn)一步推動其在航空航天領(lǐng)域中的應(yīng)用。采用硬度測試、電導(dǎo)率測試、金相顯微鏡觀察、室溫拉伸性能測試、DSC、SEM、XRD以及TEM等分析測試方法,研究X2A66鋁鋰合金在不同熱處理工藝下的硬化行為、室溫拉伸力學(xué)性能及其微觀組織變化規(guī)律,優(yōu)化出了合理的熱處理制度,探究了該合金在時(shí)效過程中沉淀析出相的動態(tài)析出行為及其演變規(guī)律。固溶處理研究表明,在460~520℃和40~80min范圍內(nèi),隨著固溶溫度的升高和固溶時(shí)間的延長,X2A66鋁鋰合金中粗大的第二相粒子逐漸回溶進(jìn)入合金基體中,固溶效果增強(qiáng)。但是,隨著固溶溫度的進(jìn)一步提高或者固溶時(shí)間的進(jìn)一步延長,合金晶粒將變得粗大甚至發(fā)生再結(jié)晶,合金的強(qiáng)度逐漸降低。實(shí)驗(yàn)中所得X2A66鋁鋰合金最佳的固溶熱處理制度為520℃/80min。通過對X2A66鋁鋰合金時(shí)效行為的研究發(fā)現(xiàn),該合金固溶處理后室溫放置具有很強(qiáng)的自然時(shí)效特性。自然時(shí)效初期合金的硬度上升很快,22h后硬度便由初始態(tài)的84.5HV升至132.4HV,之后合金硬度緩慢上升;一周之后進(jìn)入穩(wěn)定狀態(tài),硬度可達(dá)140.3HV。X2A66鋁鋰合金自然時(shí)效至穩(wěn)定狀態(tài)時(shí)的主要析出強(qiáng)化相為GP區(qū)、δ’相以及少量的魚眼狀的δ’/β’復(fù)合相粒子。合金單級時(shí)效研究表明,X2A66鋁鋰合金在165℃下時(shí)效72h能夠獲得最高的硬度和強(qiáng)度,其屈服強(qiáng)度為549.1MPa,而抗拉強(qiáng)度則達(dá)到了598.8MPa,但是延伸率僅為6.8%。165℃人工時(shí)效初期的主要強(qiáng)化來源為GP區(qū)、δ’相以及少量的δ’/β’復(fù)合相,而時(shí)效中期和后期的主要強(qiáng)化相為θ’相、T1相和少量的σ相。不同溫度單級峰時(shí)效狀態(tài)下合金基體內(nèi)的析出相種類基本一樣,在185℃高溫時(shí)效時(shí),出現(xiàn)了寬度高達(dá)100nm左右的晶界無沉淀析出帶(PFZ),其能夠?qū)е潞辖饠嗔秧g性下降,對合金的力學(xué)性能造成非常不利的影響。合金雙級時(shí)效研究表明,采用165℃/48h+130℃/24h先高溫后低溫的雙級時(shí)效工藝,可使合金在強(qiáng)度大幅度提高的同時(shí)也具有較好的塑性,其屈服強(qiáng)度為525.5MPa,抗拉強(qiáng)度為581.9MPa,而延伸率為7.9%,達(dá)到了良好的強(qiáng)塑性匹配。合金雙級時(shí)效處理后的主要析出強(qiáng)化相仍為θ’相、T1相和少量的σ相,但是T1相的尺寸大小不一,且相對細(xì)小,數(shù)量明顯增多。合金回歸再時(shí)效研究表明,采用165℃/32h+225℃/40min+165℃/24h的RRA工藝處理后合金的主要析出強(qiáng)化相與其他時(shí)效制度基本無異,但是,沿合金的晶界斷續(xù)分布著大量尺寸較小的針狀和板條狀的析出相,這種晶界析出相的不連續(xù)分布破壞了晶界的連續(xù)性,能夠大大提高合金的抗腐蝕性能。經(jīng)此制度處理后的X2A66鋁鋰合金擠壓板材,其屈服強(qiáng)度為514.3MPa,抗拉強(qiáng)度為569.0MPa,延伸率為7.1%。

X2A66 alloy is the fourth generation of novel Al-Li alloy developed independently by China, which has many advantages, such as low density, high specific strength, good formability. It is suitable for manufacturing the plane wall panel structure, can achieve the double weight loss goal of material itself and structure design. Besides, it has the advantages of good structural integrity and high static strength, in line with the future development trend of aerospace technology, so it has wide application prospects. The influence of heat treatment process on the microstructure and properties of the alloy was studied, so as to fully excavate its potential performance, and further promote its application in the field of aerospace.The hardening behavior, room-temperature mechanical properties and microstructure evolution rules of the X2A66 Al-Li alloy were investigated by hardness test, conductivity test, optical microscopy(OM), room-temperature tensile test, differencial scanning calorimeter(DSC), scanning electron microscopy(SEM), X-ray Diffraction(XRD) and transmission electron microscope(TEM). Not only this thesis optimized the reasonable heat treatment system, but also ascertained the dynamic precipitation behavior and evolution rules of the precipitation phases in the alloy during aging process.The research of solution treatment shows that the coarse second phase particles in the alloy gradually dissolve into the alloy matrix with the increase of solution temperature and solution time in the range of 460~520℃ and 40~80min, and the solution effect is enhanced. But the grains will become coarse and even recrystallize with the further increase of solution temperature or time, and the strength of the alloy decreases gradually. The best solution treatment process of X2A66 alloy obtained in this experiment is 520℃/80 min.Through the study of the aging behavior of X2A66 alloy, it is found that the alloy has a strong property of natural aging at room-temperature after solution treatment. The hardness of the alloy increases rapidly in the early stage of natural aging, which increases from the initial 84.5HV to 132.4HV after naturally aged for 22 h, then the hardness increases slowly, the alloy enters into a stable state after a week, and the hardness can reach 140.3 HV. The major strengthening phases of X2A66 alloy are G.P. zone, δ’ phase and a small amount of δ’/β’ composite phase after long-term natural aging treatment.Single-stage aging research of X2A66 alloy shows that it can obtain the highest hardness and strength while aged at 165℃ for 72 h, the yield strength is 549.1MPa, while the tensile strength can reach 598.8MPa, but the elongation rate is only 6.8%.The main strengthening sources during the early stage of artificial aging at 165℃ are G.P. zone, δ’ phase and a small amount of δ’/β’ composite phase, but during the middle and late stage they are θ’ phase、T1 phase and a small amount of σ phase. The precipitates species of the alloy under single-stage peak aging state at different temperature are almost the same, but the grain boundary Precipitate Free Zone(PFZ) whose width is up to about 100 nm occurs when the alloy is aged at 185℃. It can lead to the decrease of the fracture toughness, so causes very adverse effect on the mechanical properties of the alloy.Double-stage aging research of X2A66 alloy shows that the double-stage aging process of 165℃/48h+130℃/24 h can greatly improve the strength of the alloy while makes the alloy have a good plasticity, the yield strength is 525.5MPa, the tensile strength is 581.9MPa, and the elongation rate is 7.9%, a better match of strength and plasticity of the alloy is obtained. The main precipitates of X2A66 alloy after double-stage aging are still θ’ phase、T1 phase and a small amount of σ phase, but the size of T1 phase is different, and it is relatively small, the quantity is increased.RRA research of X2A66 alloy shows that the main precipitates of X2A66 alloy after treated with a RRA process of 165℃/32h+225℃/40min+165℃/24 h are almost the same as treated with other aging systems, but, a large number of small needle-like and lath-like precipitates are intermittently distributed along the grain boundaries of the alloy, the discontinuous distribution of the precipitates in the grain boundary destroys the continuity of the grain boundaries, and it will greatly improve the corrosion resistance of the alloy. After the X2A66 Al-Li alloy extruded sheet is treated with this process, the yield strength is 514.3MPa, the tensile strength is 569.0MPa, and the elongation rate is 7.1%.